1. Field of the Invention
The invention relates to a novel electron transport material having a 2,3′-bipyridyl group, and an organic electroluminescent device (hereinafter abbreviated as an organic EL device, or merely as a device occasionally) using the above electron transport material.
2. Related Art
In recent years, attentions are paid to an organic EL device as a full color flat panel display in the subsequent generation, and the organic EL device has been actively researched. In order to accelerate practical use of the organic EL device, reduction of voltage for driving a device and long life are an essential element, and a new electron transport material has been developed to achieve the above performances. In JP 2003-123983 A/2003 (:Patent document 1), an art is described that an organic EL device can be driven at low voltage by using a phenanthroline derivative for an electron transport material, and that an organic EL device can be driven at low voltage also by using a 2,2′-bipyridyl compound as an analog of the phenanthroline for an electron transport material in a similar manner. However, characteristics (e.g. drive voltage, emission efficiency) of the device which are reported in Examples of the above document are only a relative value based on Comparative example, and an actual measurement which can be judged to be a practically applicable value is not described. Additionally, an example where a 2,2′-bipyridyl compound is used for an electron transport material is disclosed in Proceedings of the 10th International. Workshop on Inorganic and Organic Electroluminescence (:Non-patent document 1), JP 2002-158093 A/2002 (:Patent document 2), and JP H11-514143 A/1999 (:Patent document 3). The 2,2′-bipyridyl compound described in Non-patent document 1 is low in glass transition temperature (hereinafter abbreviated as Tg), and not practically applicable. When the 2,2′-bipyridyl compound described in Patent document 2 is used for an organic EL device, the organic EL device can be driven at relatively low voltage, however, further decrease in voltage is desired for practical use. A specific compound is not shown in Patent document 3.    Patent document 1: JP 2003-123983 A/2003    Patent document 2: JP 2002-158093 A/2002    Patent document 3: JP H11-514143 A/1999    Non-patent document 1: Proceedings of the 10th International Workshop on Inorganic and Organic Electroluminescence.